Conventionally, an image readout apparatus which functions to read out information on a document by forming an image of the document onto a photo detecting array, such as a CCD (charge-coupled device) array via an optical system has been developed. There is a demand for such image readout apparatus, more specifically for use with a facsimile machine or digital copying machine, wherein the image readout apparatus have a smaller size, faster reading speed, and lower cost. As these demands have been increasing, there naturally has been an increasing demand for development of an image readout lens having a smaller size, lower cost, and improved brightness.
Japanese Laid Open Patent Application Numbers S63-135911 and H4-288512 disclose examples of conventional image readout lenses. Both image readout lenses disclosed in these publications are configured of three lens groups, with each lens group being formed of a single lens element. Because the readout lens is composed of a small number of lens elements, the readout lenses of these examples are small in size and low in cost.
Although the image readout lens disclosed in Japanese Laid Open Patent Application Number S62-135911 has a bright image due to a low F-number of 4.0, it has a narrow 36xc2x0 field of view. Therefore, this image readout lens is not suitable for general use due to the limited document size that it can handle. Furthermore, the lens disclosed in Japanese Laid Open Patent Application Number H4-288512 has large chromatic aberrations, both on-axis and off-axis. Therefore, this lens is not suitable for reading out color images.
The present invention is an image readout lens for reading out information on documents, for example. The object of the present invention is to provide an image readout lens which is small in size, low in cost, has a wide-angle field of view, and has its aberrations, including chromatic aberrations, favorably corrected so as to be capable of imaging color originals with high quality.